如何通过连线对对象排序！

Question

假设有两个List：
一个存放了五个User对象，含有id，名称之类的属性；
另一个存放了四个连线关系，fromId 和 toId，这两个值取的都是user的id
A-->B-->C-->D-->E
如何能给这五个对象排序出来！！

Answer 1

可以使用拓扑排序

import java.util.*;

// User class
class User {
    int id;
    String name;

    public User(int id, String name) {
        this.id = id;
        this.name = name;
    }
}

// Edge class
class Edge {
    int fromId;
    int toId;

    public Edge(int fromId, int toId) {
        this.fromId = fromId;
        this.toId = toId;
    }
}

public class Main {
    public static void main(String[] args) {
        List<User> users = Arrays.asList(
            new User(1, "A"), 
            new User(2, "B"), 
            new User(3, "C"), 
            new User(4, "D"), 
            new User(5, "E")
        );
        List<Edge> edges = Arrays.asList(
            new Edge(1, 2), 
            new Edge(2, 3), 
            new Edge(3, 4), 
            new Edge(4, 5)
        );
        List<User> sortedUsers = topologicalSort(users, edges);
        System.out.println(sortedUsers);
    }

    public static List<User> topologicalSort(List<User> users, List<Edge> edges) {
        Map<Integer, List<Integer>> graph = new HashMap<>();
        for (Edge edge : edges) {
            graph.computeIfAbsent(edge.fromId, x -> new ArrayList<>()).add(edge.toId);
        }

        List<User> sortedUsers = new ArrayList<>();
        Set<Integer> visited = new HashSet<>();
        for (User user : users) {
            topologicalSortUtil(graph, user.id, visited, sortedUsers);
        }

        return sortedUsers;
    }

    private static void topologicalSortUtil(Map<Integer, List<Integer>> graph, int v, Set<Integer> visited, List<User> sortedUsers) {
        if (visited.contains(v)) {
            return;
        }

        visited.add(v);